The present invention relates to a bimorph element and, more particularly, to a piezoelectric actuator using a unidirectional polarization type bimorph element.
Compact, light-weight, low-power telephone terminal devices have been recently developed, and demand has arisen for the development of highly effective drive components. Conventional drive components have been electromagnets. However, an electromagnet generates only a small force at the initial stage of its operation and requires a spring to restore the initial state, thereby degrading efficiency. The conventional drive unit using the electromagnet cannot satisfy requirements of compactness and low power consumption. For this reason, a coin processing apparatus requiring a telephone station power source results in large size and high cost.
A compact, thin, low-power drive component may comprise a known PZT (trade name) bimorph element available from Krebeit Inc. However, a conventional PZT bimorph element provides only a small displacement and a small force. A power of up to 10 mW is required to hold a switching state of a driver so as to keep the bimorph element displaced at one side upon application of a voltage thereto. In addition to these disadvantages, this bimorph element has a large hysteresis and a low resistance to mechanical impact and cannot be used in the coin processing apparatus.
In the PZT bimorph element, two piezoceramic elements, i.e., piezoelectric plates which consist of lead zirconate-titanate and which are polarized along the direction of thickness thereof are overlapped. One end of each of the piezoceramic elements is fixed. Under these conditions, a voltage having the same polarity as the polarization of the piezoceramic elements is applied to one piezoceramic element, and a voltage having a polarity opposite to the polarization of the piezoceramic elements is applied to the other piezoceramic element. In other words, an electric field acting along the direction of thickness is applied to the respective piezoceramic elements which are displaced along opposing directions in accordance with a piezoelectric transverse displacement effect, thereby flexing the free end portions thereof.
Conventional bimorph elements are classified into a parallel type element wherein the overlapping piezoceramic elements are polarized in one direction with respect to the direction of thickness thereof, and a series type wherein the overlapping piezoceramic elements are polarized in opposing directions with respect to the direction of thickness thereof. The parallel type element can provide a large displacement at a low voltage and can be used in place of the electromagnet.
However, as described above, a voltage is applied to one piezoceramic element in the same direction as the polarization of the elements, and another voltage is applied to the other piezoceramic element in a direction opposite to the polarization, thereby displacing the free ends along a given direction. Thereafter, the polarities of the applied voltages are switched to restore the initial polarization states of the piezoceramic elements. Although the above operation must be repeated, in the piezoceramic element with the applied voltage having a polarity opposite to its polarization direction, the polarization effect becomes weakened. Since a piezoelectric actuator must provide a maximum displacement, a power source voltage must be increased. However, if a bimorph element comprises two piezoceramic elements each of which has a thickness of about 0.15 mm and whose material has a field strength of 580 V/mm, when the power source voltage increases to about 80 V to 85 V, the polarization of the piezoceramic elements is disturbed, and the resultant displacement becomes very small. A practical applied voltage is limited to 1/2 to 1/3 of the polarization breakdown voltage. For example, in this parallel type element, a displacement of about 1 mm in a bimorph element having a length of 45 mm is obtained at a voltage of 30 V. In this manner, when a conventional drive circuit is used although the parallel type element is used, a large displacement cannot be obtained since the power source voltage is limited.
A polarity switching circuit must have electrical characteristics wherein a high voltage can be switched, a switched state can be held at a low power, and carriers charged by the bimorph element at the time of switching can be immediately discharged. However, none of the conventional switching circuits can satisfy these electrical characteristics.
In addition, when the bimorph element is used as an actuator for coil processing, a large impact acts on the distal end of the bimorph element as an operating end and the piezoceramic elements may often be damaged. Conventional mechanisms for protecting the elements from damage have not been substantially proposed.